Variable area, push-pull sound recording system



June 13, 1950 J. G. FRAYNE 2,511,199

. VARIABLE AREA PUSHPULL SOUND RECORDING SYSTEM Filed May 1, 1947 FIG.

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/N l E N TOR V J. a, FRA m5 r ZQRNEV 2 Patented June 13, 1950 VARIABLEAREA, PUSH-PULL SOUND RECORDING SYSTEM John G. Frayne, Pasadena, Calif.,assignor to Western Electric Company, Incorporated, New York, N. Y., acorporation of New York Application May 1, 1947, Serial No. 745,147 I 1Claim. 1

This invention relates to film sound recording and particularly to the rcording of push-pull sound tracks on photographic film.

It is an object of this invention to provide a method and apparatus forrecording a bilateral variable area, class B push-pull sound track on arecording film.

It is a further object of this invention to pro- Vide a variable arearecording system incorporating an improved light valve of simpleconstruction capable of modulating the recording light beam in a mannerto produce on a recording film a class B variable area, push-pullnegative sound track.

A feature of the invention lies in the provision in a sound recordingsystem of a single modulating device comprising three fiat conductingribbons immersed in a constant magnetic field and spaced transversely inthe recording system to substantially completely mask the recordinglight beam, the two outer ribbons being responsive to the soundmodulated currents to move together in parallel relation in onedirection while the center ribbon moves simultaneously in the oppositedirection in response to the same sound currents.

It is common practice in making an original sound track to record thesound on the recording film in the form of a push-pull sound track. Ithas been found desirable in many instances to utilize class B typepush-pull recording for the reason that this method produces, withoutthe use of noise r duction systems and apparatus, a film having a veryhigh signal to noise ratio by virtue of the fact that practically notransparent areas exist on the print except those produced by the signalcurrents which actuated the modulating device.

A bilateral, variable area, class B push-pull sound track is well knownand systems employing a modulating device incorporating conductingribbons responsive to the sound currents, have been developed heretoforefor recording this type sound track. However, in the earlier systems itwas found necessary to employ two pairs of conducting ribbons, one pairfor each track, and to employ a half-wave rectifier in the circuit fromthe sound source to each of said pair of ribbons to insure the responseof one pair to the positive half-wave and the other pair to the negativehalfwave of the sound current.

In accordance with the present invention, a bilateral variable area,class B push-pull sound track is obtained in a recording system havingoptical means producing a rectangular light beam, by means of a singlelight valve having thre flat conducting ribbons disposed with theirlonger axes parallel 'to the longitudinal axis of the film andtransversely spaced in the system to form a mask for the light beam.Connections from the source of sound modulated currents are made in amanner to cause the sound currents to fiow in the same direction in thetwo outer ribbons and in the opposite direction in the center ribbon,thus producing alternate unmasking of the light beam on opposite sidesof the center ribbon.

The invention will be more readily understood from the followingdescription when read in connection with the accompanying drawing, inwhich:

Fig. 1 is a diagrammatic representation of a sound recording systemincorporating this invention; Fig. 2 is a fragmentary enlarged view ofthe light valve of Fig. 1, showing the position of the conductingribbons with respect to the pole piece aperture;

Fig. 3 is a side elevation of the pole piece and ribbons shown in Fig.2; and

Figs. 4 and 5 are respectively enlarged negative and positive filmrecords obtained by the recording system of this invention.

' In Fig. 1, the recording system in accordance with this inventioncomprises a light source I, condenser lens system 2, light valve 3,objective lens 4 and recording film F.

Light valve 3'comprises three fiat conducting ribbons 5, 6 and 1immersed in-the constant magnetic field produced by permanent magnets '8and 9 and disposed in front of ap rture In in pole piece I l.

Light from source I is focussed by lens system 2 on aperture I0. Lens 4focusses an image of the ribbons 5, 6 and 1 on the sound track area l2of film F. It will be seen in Fig. 2 that for proper adjustment beforeoperation, the center ribbon 6 covers or masks approximately one-half ofthe aperture l0 while ribbons 5 and 1 each cover or mask approximatelyone-quarter of this aperture at the ends thereof. The three ribbons arethe same width and one-half of each of the ribbons 5 and 1 extend beyondthe ends of the aperture. For per cent modulation of the ribbons theyeach move in either direction a distance no greater than one-quarter thelength of the aperture Ill so that the ends of this aperture are neverunmasked by the outside edge of either ribbon 5 or 1.

Referring to Fig. 3, it will be seen that ribbons 5 and 1 are coplanarwhile ribbon 6 is disposed in a separate, parallel plane. Thisarrangement of the ribbons permits the center ribbon 6 to move in itsown plane in a direction opposite to th direction of movement of outerribbons 5 and l to produce alternate unmasking of the aperture I onopposite sides of ribbon 6. Separation of the ribbon planes may be inthe order of one mil. A sharp image of all three ribbons may be obtainedat the film plane by employing an objective lens 4 having a depth offield approximately the same as the distance separating the two ribbonplanes.

For the sake of clarity in the drawings, a slight separation is shownbetween the ribbons. It is apparent, however, that these ribbons may beadjusted at zero modulation to just completely mask the aperture Ill,under which condition the sound track recorded will be strictly a classB type track. If for any reason it is found desirable to provide a smallinitial opening between the ribbons, a certain amount of class Arecording will be obtained at very low amplitudes. The amount of class Arecording would not, however, exceed the width of the initial opening,that is, it would be confined approximately to the width of the exposedarea l5, Fig. 4, which appears as a clear area l6 in the positive printof Fig. 5.

Referring to Fig. 1, a microphone i1 is disposed adjacent the source ofsound to be recorded. The sound modulated signal wave from thismicrophone is amplified in amplifier l8 and supplied to the primary oftransformer 19. Lead 20, from one terminal of the secondary oftransformer I9, is connected to the upper end of each ribbon and l. Theother end of each ribbon 5 and I is connected to the lower end of ribbon6. The upper end of ribbon 6 is connected by lead 2| to the otherterminal of transformer I9.

A resistance 22 is placed in shunt with ribbon 6 and is of such valuethat the current in ribbon 6 is equal to the current in either of theribbons 5 or I.

Arrows S in Fig. 2, indicate the direction of flow of the current in theouter ribbons 5 and 1 relative to the direction of flow of the samecurrent in ribbon 6 during one part of the current cycle. The directionof flow indicated will, of course, be reversed for the rest of thecurrent cycle. Under the influence of the constant magnetic field atright angles to the direction of the current, the two outer ribbons willalways move together in the same direction and opposite to the directionof motion of the center ribbon. Arrows D, Fig. 2, indicate the relativedirection of movement of the ribbons for one part of the current cycle.

Alternate unmasking of the aperture III on opposite sides of the ribbon6 will produce exposed areas 25 and 26 on the processed negative shownin Fig. 4. Each exposed area represents a halfwave of the recordedsignal. These exposed areas will appear as clear areas 21 and 28respectively, on a processed positive print as shown in Fig. 5.

Applicants invention provides a novel method and system of apparatuswhereby a bilateral variable area, class B push-pull sound track may beefficiently recorded on photographic film by a single modulating devicedirectly connected in a novel manner to a source of sound currents.

What is claimed is:

In a sound film recording system including means for producing arectangular beam of light and means for projecting said beam of lighttoward said film with its longer axis transverse to said film, a lightvalve comprising a pair of coplanar, parallel conducting ribbonsimmersed in a constant magnetic field and spaced so that their inneredges define the narrow edges of said rectangular beam of light and athird parallel conducting ribbon disposed in said magnetic field in aseparate parallel plane to mask the portion of said light beam betweensaid first-mentioned pair of ribbons, the corresponding ends of all saidribbons at one end thereof being electrically connected together, meansforming an image of all said ribbons at the plane of said film, a sourceof modulated electric currents, an electrical connection from one sideof said current source to the free end of each of said pair of coplanarconducting ribbons and a connection from the other side of said currentsource to the free end of said third conducting ribbon.

JOHN G'. FRAYNE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 2,077,193 Wente Apr. 13, 19372,161,369 McLeod June 6, 1939 FOREIGN PATENTS Number Country Date441,293 Great Britain Jan. 16, 1936

